James' World 2

Hmmmm…the Universe is Huge!

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Although it’s shown us the Universe as we’ve never seen it before — deeper, earlier, and at longer wavelengths — it’s important to remember that the main goals of JWST didn’t have anything to do with the quality of pictures it would acquire, but rather for the science questions that they’d reveal the answer to. One of puzzles was simple: we know that the Universe is full of stars and galaxies today, whose light we can easily see, but that early on, there were no stars and galaxies, and the Universe was instead filled with neutral atoms. How, then, did all of those neutral atoms become ionized once again, enabling us to see the Universe and all the starlight generated within it?

The simple answer, of course, would have to be stars. It must be that the Universe formed stars in sufficient numbers, eventually, to produce enough high-energy (e.g., ultraviolet) light so that all of those once-neutral atoms then became ionized, allowing starlight to pass through space unimpeded. But where were those stars located? What types of galaxies housed them? And when, exactly, did all of those stars form to drive this process, known as reionization?

That was one of the main JWST science goals, and for the three years since it began science operations, we’ve been closing in on the answer. Now, at last, with a new study led by Isak Wold of the UNCOVER collaboration, we’ve got the answer: tiny, modest, but common galaxies are the culprit behind how the Universe became transparent to starlight. Here’s the cosmic story of what happened, and how we know.

The simple answer, of course, would have to be stars. It must be that the Universe formed stars in sufficient numbers, eventually, to produce enough high-energy (e.g., ultraviolet) light so that all of those once-neutral atoms then became ionized, allowing starlight to pass through space unimpeded. But where were those stars located? What types of galaxies housed them? And when, exactly, did all of those stars form to drive this process, known as reionization?

That was one of the main JWST science goals, and for the three years since it began science operations, we’ve been closing in on the answer. Now, at last, with a new study led by Isak Wold of the UNCOVER collaboration, we’ve got the answer: tiny, modest, but common galaxies are the culprit behind how the Universe became transparent to starlight. Here’s the cosmic story of what happened, and how we know.

When most of us think about the distant Universe, we think about images like the one you see above: deep field images, acquired with our most powerful space telescopes, including Hubble and JWST. It appears that what we’re seeing is stars and galaxies everywhere, limited only by the amount of time we spend observing and the capabilities of our instruments. But that’s not quite true; there aren’t stars and galaxies absolutely everywhere, as even with infinite amounts of observing time, we wouldn’t see starlight coming from all regions of space.

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This JWST field of view is focused not on galaxy cluster Abell 2744, shown here, but rather on the young, low-mass, intensely star-forming galaxies found at much greater distances behind the cluster. The cluster acts like a magnifying lens, allowing 83 young, low-mass starburst galaxies to be identified, 19 of which are shown in white diamonds here.  Credit: NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025

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Click the link below for the complete article (long article):

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